1. Field of the Invention
The present invention relates to improved structural designs and methods for their manufacture. More particularly, the invention is directed at the use of stereo lithography or other rapid prototyping techniques for the preparation of integrally formed three-dimensional truss configurations, in addition to the use of such truss configurations as disposable patterns for the production of casting molds.
2. Background of the Invention
The search for lighter and stronger load bearing designs along with improvements in stiffness and strength-to-weight ratios has been a major focus in the field of material science for many years. To date, research in this area has primarily focused on exotic metals and alloys such as beryllium and titanium alloys, and composite materials. Typical composite materials may consist of fibers (or whiskers) made of glass, carbon, polymer or metal (boron, silicon carbide or aluminum oxide). Fiber-reinforced composites were developed in response primarily to the aerospace industry's need for advanced materials with a high strength and/or stiffness-to-weight ratio.
Various honeycomb structural materials have been proposed for providing high strength-to-weight characteristics. However, such honeycomb materials typically depend on their skin for strength. Such structures also are poor in response to shear stress, and reveal poor redistribution of loads.
Recently issued U.S. Pat. No. 5,527,590 (the '590 patent) describes a lattice block material formed from a lattice of fine metallic fibers which are welded together to form a three-dimensional, continuous array of triagonal structures in the form of a series of equilateral triangles. While the three-dimensional welded wire lattice block material described in the '590 patent is believed to offer advantages over prior art structures such as honeycomb systems which rely on a stress skin, the three-dimensional welded wire lattice block material of the '590 patent has certain disadvantages. For one, the materials are cumbersome to manufacture, requiring both intricate and exacting weaving and alignment steps, and multiple welding steps. Welding also has a disadvantage in that a welding operation may change the crystalline structure of the metal wire, and thus may adversely affect metallurgical properties. Also, each weld point represents a potential defect site for crack propagation, corrosion and/or material failure.
Recent advances in stereo lithography permit the production of models of complex shape formed of wax or low melting polymers or other heat-disposable materials. These disposable materials may be formed into full size models having the exact geometry of the required finished product. The models may then be used to produce ceramic shell molds, which in turn may be used to produce unitary cast products of complex shape.
For example, in U.S. Pat. No. 5,556,590, there is disclosed an apparatus for producing a three-dimensional object from a medium capable of solidification when subjected to synergistic stimulation. The apparatus is said to comprise a container for holding said medium, a means for viewing said object during formation and for shielding the viewer from excess synergistic stimulation, and means for forming successive cross-sectional layers of structure of said object at a surface of said medium, said forming means including means for adhering each successive cross-sectional layer of structure to a preceding cross-sectional layer of structure to produce said object.
Attention is also directed to U.S. Pat. No. 5,554,336 which similarly discloses a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction.
Other related U.S. Patents of interest include U.S. Pat. No. 5,182,056, entitled "Stereolithography Method and Apparatus Employing Various Penetration Depths"; U.S. Pat. No. 5,501,284, entitled "Thermal Stereolithography"; U.S. Pat. No. 5,184,307 entitled "Method and Apparatus for Production of High Resolution Three-Dimensional Objects by Stereolithography"; U.S. Pat. No. 5,174,943 entitled "Method for Production of Three-Dimensional Objects by Stereolithography"; U.S. Pat. No. 5,133,987 entitled "Sterolithographic Apparatus and Method"; U.S. Pat. No. 5,130,064 entitled "Method of Making a Three Dimensional Object by Stereolithography"; U.S. Pat. No. 5,059,021 entitled "Apparatus and Method for Correcting for Drift in Production of Objects by Stereolithography"; U.S. Pat. No. 4,996,010 entitled "Methods and Apparatus for Production of Three-Dimensional Objects by Stereolithography"; U.S. Pat. No. 5,192,559 entitled "Apparatus for Building Three-Dimensional Objects with Sheets"; and U.S. Pat. No. 5,345,391 entitled "Method and Apparatus for Production of High Resolution Three-Dimensional Objects by Stereolithography".
Accordingly, it is an object of the present invention to make use of the recent advances in stereolithography, as applied to the preparation of an integrally formed high-strength light-weight three-dimensional truss structures, and to employ such integrally formed three-dimensional truss structures for the production of casting molds, so that said three-dimensional truss structures can be readily manufactured by investment casting procedures.